1. Field of Invention
The present invention pertains to image setting and, more particularly, to an apparatus and method for providing high quality images on photosensitive material.
2. Background of Invention
To enable one skilled in the art to appreciate the context and the significance of the improvements involved in accordance with the present invention, reference may be made to the following background information, specifically to the following U.S. patents:
U.S. Pat. No. 3,687,025--Rosin PA0 U.S. Pat. No. 4,284,994--Radl
From the references cited, it will be appreciated that a variety of printers, including those classified as dot matrix printers, have already been developed. Most of such printers have typical dot matrix resolutions of 100 dots per inch or less, although some of them apparently provide 300 dpi, which is considered high quality for printers. On the other hand, laser phototypesetters can provide writing resolutions between 720 and 2,500 dpi, with the bulk of them having resolutions lying in the range of 975 to 1,200 dpi.
Nevertheless, what has heen lacking in the field is a cost-effective apparatus for achieving the requisite high degree of resolution in image setting.
It is therefore a primary object of the present invention to provide a low-cost, high-resolution, image setter for reproduction-quality imaging.
Another, more speciiic object is to facilitate the production of output images having widths equal to or greater than 72 picas (12 inches).
Another specific object is to set images at resolutions ranging up to 2,400 dpi and at a speed of over 170 rasters per second, which corresponds to one inch of output every seven seconds at 1,200 dpi. and proportionately shorter times at coarser resolutions.
It is well known that laser typesetters utilizing multifaceted mirrors, in conjunction with a conventional optical system, require that each and every facet be corrected individually to match each other due to the practical limitations of facet manufacturing tolerances.
In this regard, particular reference may be made to the aforementioned U.S. Pat. No. 4,284,994. The invention described therein relates to laser beam recorders or printers using a polygonal mirror as a scanning device, the improvement being particularly directed to an optical system for compensating tilted facets of the polygonal mirror.
The U.S. Pat. No. 4,284,994 does generally set forth a scheme or system involving a laser beam containing input information, and including means for reflecting that beam through a scan lens system, a polygonal mirror functioning to produce a scan beam with constant angular velocity. The scan lens acts in response to the scan beam to provide an image point that moves across the film with a constant velocity. Concurrently or alternately with the movement of the scan beam, a mechanical scanner moves the film either continuously or in discrete steps, thereby to provide the other dimension of the requisite two-dimensional image.
Conventional laser typesetters have generally used gas lasers, such as helium-neon lasers, in their operation. Not only are such lasers, with their accompanying power supply, modulator and modulator driver relatively expensive, but the helium-neon laser, in particular, suffers from the fact that the peak modulation rate is limited by the size of the output beam. Only with further optics improvements, involving additional cost, can the modulation performance be improved.
In contrast thereto, currently available laser diodes in the milliwatt range can readily be modulated in an image setter at rates of at least 20 MHz. Accordingly, it is a primary object of the present invention to judiciously exploit, by a unique combination of apparatus, the capability of such laser diodes with high modulation rates.
Another more specific object is to achieve an efficient compromise between speed and alignment requirements in producing high quality images on photosensitive material.
Yet another specific object is to enable more efficient use of the laser light energy available by providing that the prism mirror involved in the optical arrangement achieves a relatively longer dwell time on the photosensitive material than conventional systems.
In previously known phototypesetters a serious drawback involved the structure of the apparatus for receiving the image and transporting the film. Frequently the prior art mechanisms for film transport involve a relatively long film path from the point at which the film is removed from a film supply cassette and moved past the optical system to the point at which it is rewound on an output reel or the like.
It is therefore another primary object of the present invention to eliminate the wastage in photosensitive material involved in the long path from input to output points.